The processing of crude petroleum gives rise to various hydrocarbon fractions which may be subsequently "cracked" by heating, usually in the presence of steam to produce a range of lower boiling products.
Many of the commercial applications of petroleum resins have over recent decades required that the resins be increasingly lighter in color to colorless.
For aliphatic to predominantly aliphatic feedstocks which are cationically polymerized, it was early reported in U.S. Pat. No. 2,734,046 that light colored resins can be produced from unsaturated predominantly aliphatic petroleum fractions obtained by steam cracking when the fraction is substantially free of cyclic dienes. It is thus known that these cracked predominantly aliphatic fractions contain cyclodienes which contribute to the formation of gel and adversely affect the color, clarity and other properties of the resin. U.S. Pat. Nos. 2,894,937 and 2,946,775 report that it is a simple matter to dimerize these cyclodienes by a thermal soaking treatment and to separate the dimerized hydrocarbons from the remainder and to use this remainder as the aliphatic feed for polymerization by aluminum chloride. Although the resulting resins were of lighter color and had desirable softening points, the colors obtained were still too dark for many industrial applications necessitated by technological developments in the pressure sensitive adhesives into which petroleum resins are incorporated. Resins of much lightened color were recently reported obtained (see U.S. Pat. No. 4,391,961) by heat soaking a petroleum cracking or reforming fraction containing cationically polymerizable hydrocarbons including from more than 0.5 to 2 weight percent of cyclodienes until the cyclodiene content is no more than 0.5 weight percent and the piperylene to cyclopentadiene weight ratio is above at least 50, distilling said heat soaked petroleum fraction, polymerizing said distilled petroleum fraction in the presence of an aluminum halide catalyst and recovering an aliphatic resin having a Gardner color of no greater than 4.
Unfortunately, the polymerization of an aromatic feedstock boiling between 80.degree. C. and 260.degree. C. and containing C.sub.8 to C.sub.10 olefinic aromatics such as styrene and indene, with a Friedel-Crafts catalyst such as AlCl.sub.3 produces a dark colored resin (Gardner color of 11 to 13 based on a 50% solution in toluene). The literature reports improving the color and thermal stability of aromatic hydrocarbon resins by reacting an aromatic olefin feed stream with a dienophile, e.g. maleic anhydride, prior to polymerization (U.S. Pat. Nos. 4,105,843 and 4,230,840).
One approach to producing paler colored resins from a given petroleum fraction feedstock is to subject the feedstock to a concentrated sulfuric acid refining step followed by redistillation of the acid treated feedstock prior to its polymerization in the presence of a Friedel-Crafts catalyst into a petroleum resin (see U.S. Pat. No. 3,042,660, col. 1, lines 63-71).
It thus appears that it is more difficult to provide a light colored petroleum resin from aromatic olefin feedstock than it is from aliphatic olefin feedstock.
It is therefore an object of this invention to provide aromatic petroleum resins of much lighter color and having a suitably high softening point than presently known and/or available.